The present invention relates to an electrode for electrochemical processes, particularly for the electrolysis processes to produce chlorine, consisting of a metal resistant to the electrolysis medium and an active cover layer applied thereon, said layer containing the substances producing the electrode process.
Technical anode materials have to meet a number of requirements. For reasons of durability, the anode material must not only be sufficiently corrosion-resistant, but it must also be possible to carry out the anode process at sufficiently high speed. In the case of coated electrodes, the electron conductivity of the anode core and the surface layer, due to energy reasons, must be high and the excess voltage of the anode process must be low. Possible corrosion products of the anode may not disturb the normal operating course of the electrolysis process. Furthermore, the anode material should of course be inexpensive.
The conventional electrodes only partially meet these stringent requirements.
Platinum, platinum metal and their alloys have for a long time been known as resistant electrode materials. For example, the first horizontal mercury cells for electrolytically obtaining chlorine and soda lye were equipped with anodes of platinum and platinum/iridium. On account of the high costs for equipping with platinum wire anodes and last but not least also on account of the considerably high corrosion rates of the valuable precious metal -- the specific platinum loss was, even with the still very low plate current densities of that time, already at between 0.3 and 0.6 g platinum per ton of chlorine produced -- it soon became necessary to change over to the more economical graphite anodes.
The idea to coat with platinum a non-precious base metal, such as copper, iron etc., in order to thus attain an inexpensive anode material is also already quite old. However, in the field of chlorine, alkali electrolysis, these platinized metal anodes very soon succumbed to the corrosive effect of the cell media.
Finally over the past few years anodes have become known, the base body of which consists of a non-precious metal that can be passivated, for example titanium, tantalum, zirconium, niobium, and the active cover layer of which consists of a platinum metal or a platinum metal alloy. It has also meanwhile become known that these metal anodes as well as their plated predecessors are not suited for use in amalgam cells. For their expensive precious metal plating in the long run cannot cope with the numerous intensive loads of mechanical, electrical, chemical and electrochemical nature that are prevalent in the present-day giant cells. The effectiveness of the platinum metal layer kept thin for reasons of cost very soon deteriorates which on the one hand results in constant voltage increases and on the other hand requires frequent anode exchange. A sudden failure of the entire metal anode line-up will necessarily have to be expected in the case of the horizontal mercury cell which has meanwhile found wide acceptance; it is well known, that with it the danger of a short circuit is particularly great and any contact in it with the mercury will immediately remove the platinum metal of the coating due to the fact that it readily amalgamates, thus making the anode inactive at this location.
Moreover, electrodes have become known, the cover layer of which consists of binary oxides or of the mixture of binary oxides of the platinum metals. Moreover, this cover layer can also still contain up to one-half the amount of the platinum metal oxides provided other oxides that are difficult to reduce or that are refractory. It is a known draw-back of these electrodes that the binary precious metal oxides which are known to readily reduce are reduced to the respective metal in mercury cells on account of the amalgam, so that these anodes involve the same draw-back as the already described anodes that are provided with a cover layer of platinum metal. It is a completely logical consequence that this effect occurs with anodes that are coated with oxides of the platinum metals, which is due to the fact, also already known for a long time, that the platinum metals are also covered with electrically conductive oxidic cover layers during use of the anode. It is unimportant as to whether these oxidic cover layers are formed before or only during use in electrolysis. For example, no improvements were attained with any anodes having cover layers the essential component of which was binary oxides of the platinum metals or their mixtures but, rather they involved the same draw-backs as are known for the anodes having cover layers of platinum metals, which are already known much longer.
Moreover, anodes have become known having cover layers in which the oxides of the platinum metals and the oxides that are difficult to reduce or that are refractory, particularly titanium oxide and tantalum oxide, occur in exactly the reverse combination ratio as in the above mentioned anodes. It is known that titanium dioxide and tantalum pentoxide are excellent electrical insulators, which are entirely unsuited for carrying through an anode process. Although the electrical resistance of said materials can to a certain extent be reduced by doping or by oxygen removal, these materials, however, cannot at all be termed good electrical conductors. A further, even graver draw-back is that oxides of titanium or tantalum that have been made conductive by oxygen removal become good insulators again under oxidizing conditions, as are usual with an anode, which means they become bad electrical conductors.
Furthermore, anodes have become known which have a cover layer containing substances of the type Me(I).sub.ca. 0.5 Pt.sub.3 O.sub.4 or consisting of said substances. Although good results have been achieved with this anode when used in electrolysis processes operating with high current load and permitting renewal of the anode coating only at longer time intervals, certain difficulties arise in the manufacturing of anodes to be used in electrolysis processes operating with low current load or, respectively, when the anode coating is renewed within shorter time intervals, because in the latter case very thin cover layers are concerned or, respectively, cover layers containing little Me(I).sub.ca. 0.5 Pt.sub.3 O.sub.4. These difficulties were soon recognized and it was suggested to correspondingly increase the electrical activity of the cover layer by materials improving the conductivity, for example carbides, borides or nitrides of titanium, tantalum, zirconium, niobium or mixtures thereof. In such a case, the electrode process is always caused by the substances of the type Me(I).sub.ca. 0.5 Pt.sub.3 O.sub.4. On account of the particular preparation, the substances of the type Me(I).sub.ca. o.5 Pt.sub.3 O.sub.4 are incorporated as solids into the cover layer, and the electrode whose cover layers are made up with materials improving conductivity of the type just described exhibit properties not entirely satisfactory for some electrolysis purposes.